The present invention relates generally to x-ray imaging, and more specifically to methods for enhancing image quality in digital x-ray images for dual energy imaging.
The classic radiograph or xe2x80x9cx-rayxe2x80x9d image is obtained by situating the object to be imaged between an x-ray emitter and an x-ray detector made of photographic film. Emitted x-rays pass through the object to expose the film, and the degree of exposure at the various points on the film are largely determined by the density of the object along the path of the x-rays.
It is now common to utilize solid-state digital x-ray detectors which comprise an array of switching elements and photosensitive elements such as photodiodes in place of film detectors. The charge is generated by the x-rays impinging on the various points of the detector array and are read and processed to generate a digital image of the object in electronic form, rather than an analog image on photographic film. Digital imaging is advantageous because the image can later be electronically transmitted to other locations, subjected to diagnostic algorithms to determine properties of the imaged object, stored, and so on.
During the digital imaging process, the image is generally not produced directly from the detector reading. Instead, the detector reading is processed to produce a cleaner image. In particular, the image is usually processed to eliminate the xe2x80x9coffset,xe2x80x9d which is the image which would be obtained in the absence of exposure. The offset is determined by the detector leakage current, temperature, background radiation and a variety of other factors. The offset is desirably eliminated from the detector reading to provide better image quality. One method of capturing an image and subtracting its corresponding offset is disclosed in U.S. Pat. No. 6,115,451 entitled xe2x80x9cArtifact Elimination In Digital Radiographyxe2x80x9d which is commonly assigned to the assignee of the present invention and is herein incorporated by reference.
Dual Energy Subtraction (xe2x80x9cDESxe2x80x9d) is an important clinical application of digital x-rays imaging. DES consists of acquiring two x-ray images at different energies, and creating an output image by a combination of the two images. Using a solid-state digital x-ray detector, two acquisition frames are required; one image is acquired at a relatively high average x-ray energy, and the second image is acquired at a lower average x-ray energy. The time between the two image acquisitions is used to modify the x-ray energy. To minimize motion artifacts from respiration, heart motion, or other physiological motion, it is desirable to minimize the time duration between the two x-ray exposures. In addition, radiographic imaging with a digital detector requires the subtraction of an offset frame with similar acquisition timing as the x-ray frame.
Thus, there exists a need for a robust method of acquiring digital radiographs for Dual Energy Subtraction applications which includes a method for acquiring the corresponding effect and x-ray images.
It is an object of the present invention to provide an improved methodology for digital radiographic imaging in Dual Energy Subtraction applications. It is also an object of the present invention to provide a method for minimizing motion artifacts in Dual Energy Subtraction digital radiographic imaging applications.
The foregoing and other objects are provided by a method of radiographic imaging which minimizes the time lapse between the two x-ray exposure frames. This is accomplished by acquiring the two x-ray exposure frames relatively consecutively without a corresponding offset frame reading in-between the two x-ray frame exposures. Preferably, the offset frames are acquired following the x-ray exposure frames with a corresponding timing sequence which is correlated to the x-ray frame exposure and reading sequence. In preferred embodiments of the invention, the method includes the steps of exposing a radiographic detector at a first average energy level for a time period t1; reading the radiographic detector to obtain a first exposure reading; exposing the radiographic detector at a second average energy level for a time period t2; reading the radiographic detector to obtain a second exposure reading; after a time period equal to t1, reading the radiographic detector to obtain an offset reading; and subtracting the offset reading from the first and second exposure readings.
In another embodiment of the present invention, first and second offset readings are taken in a timing sequence corresponding to the timing sequence of the first and second exposure readings to ensure that the conditions under which the offset readings are taken closely resemble the conditions under which the x-ray exposures were taking thereby minimizing artifact-related errors. In a preferred embodiment, the exposure time of the first x-ray is greater than the exposure time of the second x-ray and a delay period (t3) is added prior to the second exposure reading to make the total time period prior to the second exposure reading (t2+t3) equal to the total time period prior to the first exposure reading (t1). The offset readings are then taken after a similarly spaced timing sequence between the two offset readings. In another aspect of the invention, a single offset reading is subtracted from the first and second exposure readings. In another aspect of the invention, a plurality of offset readings are taken, and the average of the offset readings is subtracted from the first and second exposure readings.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference of the accompanying drawings.